Process of casing steel articles



Patented Mar. 26, 1935 UNITED STATES PATENT OFFICE j No Drawing. Application November 21, 1931,

SerialNo. 576,618

3 Claims.

This invention relates to forming thin hard casings on steelor iron articles, or alloyed steels.

By the novel process, the surfaceportions of the articles are converted into casings which are distinguished by being much harder than have been producible by mere carburization of the articles without quenching, and further distinguished by the characteristic that the hard casing is not brittle, and does not tend to flake or chip ofi Among other uses, the novel process may be employed as a substitute for the common process of case-hardening by means of cyanide. Cyanide fumes are very poisonous. They destroy equipment, furnishings, etc. Moreover, a cyanided article has to be quenched, whereas according to the present invention the desired super-hardness can be obtained without quenching. Moreover, cyaniding has a more restricted field than has the present inention, since when it is attempted to proceed with the cyaniding to a depth of substantially more than five thousandths of an inch, the objection of flaking arises; moreover its hardness decreases to the point of inferiority.

The heavy casing which was produced by simple nitriding of an iron, lacked the required backing. There was no element which eifected a permanent union between the nitrided shell and the body of the article. The nitrided portion was too thin and too hard, and was in contiguity with the unaltered metal of the article, which had soft-. ness and other substantially different characteristics from the nitrided casing, and cleavage was apt to occur.

Such cleavage or flaking has proven to be a serious fault, prohibitive of the practical use of nitriding for ordinary steel. It has been proposed to minimize the fault by employing steel alloyed with aluminum or other metals, but this has confined nitriding to a very restricted field, using low heats, and the cost has been excessive, on account of the length of the treatment. The desired quality could not be attained by nitriding even the aluminum steel and other special alloy steels. The resulting surfaces were excessively brittle. and harder than desired in many cases.

In nitriding, the articles were maintained at the required heat for days, involving great cost for fuel. The slowness of the nitriding process demanded a great increase of equipment, which consumed excessive floor space. It was sought by the nitriding process to overcome the faults of carburizing. Carburizing has been performed for example in the apparatus disclosed in patent granted to me April 20, 1920, No.

1,337,244, and others, the articles being carburized to the desired depth; but said casing was not sufliciently hard, so that it was usual to quench the carburized articles, in order tobring them up to the required hardness. In quenching carburized articles, distortion thereof was often unavoidable.

Moreover the process requiring the articles to be maintained for many hours at very high heats, and involving heavy expense for fuel, and also necessitating the use of bulky and expensive equipment, which was subject to deterioration from the high heat of operation.

It was not found practicable to attain extreme thinness of carburizing.

A steel article can be'nitrided only to a slight depth. The thinness of the casing is-a characteristic of the present invention, which therefore belongs in the art of nitriding, but the casing can be made considerably thicker than a simple nitride casing, and other faults are overcome.

The nitriding process according to the present improvements can be carried out in any suitable apparatus, as for example in the machines of the types disclosed in said patent.

In nitriding according to the present invention in one way, the articles are maintained at the proper heat in the retort while immersed in a current of ammonia (the source of nitriding) but the ammonia is diluted by hydrocarbon gas. Three to five volumes of ammonia gas are diluted by one volume of any carburizing gas which is lean in carbon-as is common in the art. The diluting gas may be ordinary city gas,.or natural gas, or butane, methane, or other gas having a moderate proportion of carbon. If too richin carbon, the carbonous gas may be first thinned out in a known manner, to render it suitable for use as a diluent for the ammonia in practicing the present invention. The ammonia gas may be supplied through one pipe, and the carbonous gas may come through another pipe; the flow of each gas being separately controlled and its volume measured by a flowmeter. One pipe delivers a measured volume or proportion into the other, and the diluted nitriding gas mixed with the diluent in the proper proportions is led to the heated retort containing the iron or steel articles to be nitrided in the improved manner.

The proportions of the ammonia gas and the carbonous gas may be varied, depending upon the metal that is being treated, and upon other requirements. The described diluted or mixed gas,

when heated, will penetrate some metals or alof carburizing was slow,

loys at a substantially lower heat than other metals.

One of the characteristics of the invention is that the articles do not need to be heated to the high point used in carburizing, which in practice is about 1600 to 1750 degrees. This characteristic of the lower heat, aids in distinguishing the present improvements in nitriding from ordinary carburizing.

Another of the dstinguishing characteristics is that the improvement in nitriding is not practiced at low nitriding heat. When a steel article was exposed to ordinary ammonia gas, the metal absorbed nitrogen if the retort was heated up to 850 to 900 degrees F. But the process of nitriding according to the present invention is not practiced at the low point of 900 degrees.

The articles, being brought to the required heat, are exposed to a current of the described thousandths of an inch, so that it belongs in the nitriding class, whereas an article may be cased to a depth of inches, in well-known carburizing practice.

To obtain the desired result promptly, the heat of the articles may be maintained at 1350 degrees Fahrenheit or above. Excellent results are obtainable at a heat of 1400 degrees. The speed of the improved nitriding is increased if the temperature is raised to 1450 degrees, but the temperature is kept below the crystallizing 'or critical point.

The improved nitriding is done much more readily and the processing operation is under better control, as compared with ordinary carburizing. Nitriding 'has not been practiced at the high heats used for ordinary carburizing. Neither has carburizirig been practiced at the low heats which have been used in nitriding steel. It may here be observed that to carburize articles at as low a temperature as 1300 or 1350 degrees has never been found to be practicable, and would have required the articles to be kept heated for a very long time at great expense. But the presence of the carburizing gas as a diluent of the ammonia gas in the retort, is thought to have the effect of substantially lowering the point of heat at which the steel can be cased as compared with carburizing, while raising the effective point as compared with simple nitriding.-

The proper working temperature for the novel process is hundreds of degrees below the old carburizing temperature, and hundreds of degrees above the old nitriding temperature. This would be from 1350 to 1450 degrees, or about midway between the old carburizing and nitriding temperatures.

' The exterior portion of the treated article is converted into a homogeneous carbonous iron nitride, which exteriorly is glass hard, but substantially less hardthan nitrided steel, although it is substantially harder than unquenched carburized steel, and possesses the characteristic of be converted in three minutes. Somewhat heavier articles require five minutes.

The finished coating may penetrate the five thousandths of an inch in five minutes after the articles are heated to the required point. In one kind of metal the penetration will be deeper than in another kind.

One of the valuable features of the improved nitriding is the great reduction in cost. A great saving is effected by the described reduction of the time at which the heat has to be maintained at a maximum. The saving of time is very great as compared with the old simple nitriding. The cost ofencrusting the articles is very much less than the cost of nitriding articles according to old practice. Moreover, as compared with carbu 'provements in nitriding hardness is secured without quenching, but the. described hard finish or crust can be secured by this invention without distorting the article. Distortion of carburized articles is mostly due to quenching them, in order to secure the required high degree of hardness. But there is no need to quench articles which are nitrided by the present invention; hence there is no warping or distortion. The articles may cool slowly.

Nitriding has been practiced at lower heat than the present invention, but it was necessary to maintain the heat a very long time. The depth to which nitriding penetrated depended upon the metal being nitrided. Even when the article was maintained at nitriding heat for an hour the depth would only reach about five thousandths of an inch.

It is therefore evident that the present nitriding invention, whereby a depth of hardness of five thousandths of an inch is obtainable in five minutes, is distinct from and animprovement over the process of simple nitriding.

It is also evident that a difference is due to the modification effected by the presence of carbon, and that the casing or shell includes nitrogen and carbon with the iron.

The invention has steel and carbon and nitrogen in a casing which hardens without quenching, and it may be made of much greater thickness (say an increase of ten-thousandths of an inch) than is feasible with nitriding, thereby establishing that there is a substantial ingredient of carbon in its structure.

It is evident that carbon in some condition is one of the constituents of the casing of this invention. First, because the penetration may go much deeper than would be found practicable with simple nitriding. The extra depth is evidently due to the carbon. Second, because the case coheres to the body of the article and forms a substantial integral body therewith, making the casing, even when it is quite thick, practically inseparable from the body of the article; whereas simple nitriding could not proceedto such depth, and not even to five-thousandths without separating and flaking ofi. The flaking of the old nitrided article may-start when it is attempted to reheat the article, 'or to work it in various'ways, or to subject it to heavy use.

According to this invention the material of the casing and the material of the body merge essed undergoes conversion due to the joint action of the carbon and nitrogen, or of the carbonous gas and ammonia gas, upon the iron or steel substance, whereby the latter is converted into a hard casing which is integral with the body of the article. I

This crust so formed is substantially harder than common unquenched case-hardening, but not so hard as if the article had simply been nitrided according to the old process. The hardness is the joint result of the action of the iron,'

carbon and nitrogen upon one another, or to the joint effect of the iron, carbon, nitrogen and hydrogen upon one another. The surface isglass hard, or cannot be worked by a file, but by terminating the heating before undue hardening occurs, it results that the novel nitrided casing gradually merges into the softer body of the article. The hardest condition is at the exterior or surface, and this hardness diminishes gradually or in other words thereis merging of the hard-surface shell to the relative softness of the body of the article.

The substance into which the iron is converted may be composed of some iron, and also some iron combined with nitrogen and also some iron combined with carbon, or it may be iron which is combined throughout with nitrogen modified by carbon or by carbon and hydrogen.

In diluting the ammonia gas with carbonous gas there is brought into existence in the final structure a carbonous bond which permeates the nitrided shell or casing, and also penetrates into the article, to render the casing permanently integral with and practically inseparable from the body. It is thought that the carbonous gas (which penetrates iron more readily than does nitrogen) may serve to carry portions of the nitrogen more deeply into the body of the article than would be the case if nitrogen alone were used, according to prior simple nitriding process.

q It is thought that the structure exhibits a homogeneous casing of nitrocarbonous alloy of iron 'in solid andinseparableor non-flakingunion with the body of the article.

The articles encased according to this process are capable of withstanding severe usage without danger to the casing. In many instances the cased article can be given a bend of degrees without even showing a crack, this result being due to the malleable quality of the novel casing. If an article, which is cased according to the present invention, should then be bent more sharply, enough to exceed the limit of malleability, it might result in cracking the casing. Notwithstanding it then cracks, the casing does not flake off, as it is too strongly combined with or knit to the body. I

By this processing a surface may be obtained,

' which, while very hard, still does not possess the objectionable hardness which results from nitriding aluminum steel and other special alloy steels.

The invention enables ordinary steels to be encased, not being restricted to special alloy steels.

The novel casing is very strong or tough. The articles may be hammered and thereby peened or elongated or caused to flow, without flaking on the casing, or even showing any signs of flaking.

-Although not quenched, it is as hard as some quenched carburized metal, and may be even harder. With some steel articles, the novel unquenched nitrided 'casing is harder than can be produced by carburizing and quenching.

This processing can be used universally for giving an inexpensive surface hardening to all kinds of devices. The hardening not only pro- .duces wear-resistance, but also stiflens the articles in many cases. Generally this process may be used wherever it has heretofore been the practice to use cyanide.

One of the practical advantages of the inven- 20 tion is that it can be used for treating articles that are made of a high carbon steel. There,

can'be used 10, 20, 30, and even' up to 90 point carbon, and one percent carbon in alloyed steel articles where great strength is required, to afford a strong support for the hard casing.

The present processing will answer for practically all grades of carbon and alloyed steels. It is not limited to any particular point or proportion of carbon in the steel article. It is for general use. Where the invention is used for hardening a bearing, for example, the encased bearing may have a steel body which is made with sufiicient carbonto insure the requisite strength of the bearing, and avoid liability of flowing or indentation of any part of the bearing, under the pressure of the journal to which the bearing is fitted. Thus a hard surfacing ,which has a depth of only five-thousandths of an inch may suffice for bearings designed for relatively heavy service. v

The old nitrided casing is not readily annealed if at all, but the casing of this invention can be annealed under high heat, say 1500 to 1600 degrees.

The articles encased by the present invention may be quenched in water if desired, so as to produce a still harder surface.

The processing may be carried out in a modified way and with a modified result. According to the modified process, about three volumes of ammonia gas may be diluted by about three volumes of lean hydrocarbon gas, to form an atmosphere in which the heated articles are confined at the processing operation. The heating operation may be longer than heretofore described, say twenty to thirty minutes. The result has the additional distinguishing characteristic, in that the'surface of the nitrided articles is found to be graphitic. This graphite finish may be left without further treatment; or, if desired, will be found to take an excellent polish. Whether polished or not, the graphitic' surface is self-lubricated, and also gives protection or union joining the surface and body portions of the article is very efficient.

The invention may be utilized, on account of its -finish is wanted, and more especially when a graphitic finish is desired.

Variationsmay be resorted to within the scope vo! the invention, and portions of the improvements may be used without others.

Having thus described my invention, I claim:

1. The process of converting the surface portions of steel or iron articles into a hard malleable case, including maintaining the articles at a converting heat of about 1350 degrees F. in a retort while immersed in a fresh current of .a preparation of ammonia and hydrocarbon gas, which is neutral to the articles at 900 degrees F. or less, the proportion being three volumes of ammonia gas and one volume of hydrocarbon gas.

2. The process of converting the surface portions of steel or iron articles into a hard malleable case, including maintaining the articles in a retort while immersed in a fresh current or a preparation of ammonia and hydrocarbon gas, which is neutral to the articles at 900 degrees F. or less, the proportion being three volumes of ammoniagas and one volume of hydrocarbon gas, the retort being kept at a heat or about 1300 to 1450 degrees F.

3. The process of converting the surface portions of steel or iron articles into a hard malleable case, including maintaining the articles at a converting heat in a retort while immersed in a fresh current of a preparation or ammonia and hydrocarbon gas, which is neutral to the articles at 900 degrees F. or less, the proportion being three volumes of ammonia gas and one volumeoi hydrocarbon gas, the retort being kept at a heat of about 1300 to 1450 degrees F., and the duration oi the exposure of the heated articles to the described gas being irom about three to about five minutes, producing a case of about five-thousandths of an inch depth, the preparation of gas penetrating to a greater depth into the body of the article than is attained by simple nitrogen, the length of the process being terminated at a point to prevent undue hardening or other deterioration of the casing or separation.

ADOLPH w. MACHLET. 

